The home life of a resident of a modern industrial city is oversaturated with household appliances: audio, video, kitchen, computer, etc. Much of what the city dweller now cannot (or does not want to) do without is a rather powerful source of radio interference, due to which the reception of even powerful broadcasting stations is either very difficult or impossible at all, instead of “live” ether, only a monotonous low-frequency rumble and crackle is heard, regardless of the frequency to which the radio is tuned. This is especially noticeable in the ranges of long and medium waves. The article “Suppression of Common-mode Interference to Radio Reception” not only outlined a method to suppress such interference, but also described the design of a layout of a symmetrical ferrite magnetic antenna for the medium wave range, which allowed the author to receive signals from far distant broadcasting stations in an apartment of a reinforced concrete panel house. Below we will talk about how to make such an antenna from some old radio.
The operating experience of the described antenna, as well as a number of reviews from those radio amateurs who also tried a similar antenna in operation, revealed the following features:
But the design described in the article does not allow to fully use these advantages of a symmetrical ferrite magnetic antenna. In order, for example, to tune out interference that interferes with radio reception, using the directional properties of the antenna, it often had to be not only rotated in a horizontal plane, but also tilted. And the sharp resonance of the antenna oscillatory circuit makes it very difficult to fine-tune to the required frequency.
After reflecting on the above, the author came to the conclusion that if a symmetrical ferrite antenna is made compact and light, then for its free orientation in space it is very convenient to use some kind of photo tripod, and a vernier mechanism with a scale can greatly facilitate accurate and quick tuning of the antenna in resonance with received signal. So the idea arose to make an anti-noise ferrite rod antenna from the parts of an old transistor radio.
A slightly disassembled SELGA-404 radio receiver, produced somewhere in the mid-1970s, turned out to be at hand. Its printed circuit board, a ferrite rod with loop coil frames, a variable capacitor, two trimmer capacitors, as well as part of the front panel with a scale and details of the vernier mechanism were useful as a “kit” (see photo in Fig.1).
The procedure for manufacturing the antenna is as follows. First of all, we remove the aluminum disk with an arrow and, looking at the wiring of the cable between other parts of the vernier mechanism, we draw its kinematic diagram on a piece of paper by hand (see photo in Fig.2). If this is not done, then the reassembly of the vernier mechanism at the final stage of manufacturing the antenna can be very delayed.
After that, we remove other parts of the vernier mechanism and completely free the printed circuit board from the rest of the elements, and at the same time, if possible, remove excess solder from the board (see photo in Fig.3).
For the manufacture of the antenna, not the entire printed circuit board is needed, but only that part of it where a variable capacitor with a vernier mechanism is installed. This part of the board fits in width to the fragment of the front panel of the radio receiver case with the tuning scale shown in Fig.1. With some sharp tool we mark the cut line on the printed circuit board and saw off the excess with a hacksaw, as shown in Fig. 4. The translational movements of the hacksaw should be smooth, without jerks, since the getinaks, on the basis of which the printed circuit boards of such receivers were made, is a very fragile material and it can crack from a sharp movement with a hacksaw.
From the sawn off unnecessary piece of the printed circuit board, we remove the second holder of the ferrite rod (see photo in Fig.5).
We make a bracket from a duralumin corner. Then a board with elements will be screwed to one of its planes, and the other plane will serve to attach the entire product to a photo tripod. On the same bracket we install an antenna connector for connecting a coaxial cable. Using a knife, we remove unnecessary printed conductors from the printed circuit board itself, drill the missing mounting holes, and where the bracket will be attached to the board, cut out part of the board for the antenna connector with a jigsaw or file. Then we insert tubular rivets in part of the holes in the places where the elements are supposed to be installed (see photo in Fig.6). This completes the preparation of the printed circuit board for installation.
The electrical circuit diagram of an active anti-noise ferrite rod magnetic antenna is shown in Fig.7. The signal to be fed to the antenna input of the radio receiver is taken from the secondary winding of a symmetrical matching high-frequency transformer T1:III. The two primary windings of this transformer T1:I and T1:II are connected to the sources of JFETs VT1 and VT2, on which a differential push-pull source follower is made. Resistor R1 sets the operating DC mode of JFETs and serves as a load for the common-mode component of the output signal.
The ferrite magnetic antenna itself is an oscillatory circuit tuned to resonance with the received signal, the inductors of which, indicated in the diagram as WA1:(I-II), are located on a ferrite rod. To tune the circuit to the frequency of the received signal, a two-section variable capacitor C1 is used. Trimmer capacitors C2 and C3 are necessary for setting the frequency boundaries of the range of received signals and setting the symmetry of both of two parts of the oscillatory circuit.
The signal selected by the antenna oscillatory circuit is fed to the input of a differential push-pull source follower, the gates of JFETs VT1 and VT2, through a symmetrical coupling coil WA1:(III-IV). Such a connection of the amplifying stage to the antenna oscillatory circuit makes the circuit insensitive to low-frequency interference, which can parasitical modulate the useful signal.
If you use any AC-to-DC-adapter to power the described circuit, then it can unexpectedly become another source of radio interference. The reason for this is the widespread use of switching power supplies, which are now equipped with almost all computer and household appliances. A special filter is designed to prevent the penetration of high-frequency interference into AC mains, with which such power supplies must be equipped. But many radio amateurs and specialists in the repair of such equipment are well aware that manufacturers, in pursuit of the low cost of their equipment, often do not install such filters at all. Therefore, it is better to install a filter that suppresses high-frequency noise in power circuits yourself. In the circuit diagram in Fig.7, elements C5-L1-C6 serve this purpose.
Capacitor C4 reduces the effect of possible low-frequency noise on the power supply, and switch SA1 will be especially useful when powering the circuit from batteries. The elements VD1, VD2 and R2 will be discussed below.
Before you start mounting the elements on the circuit board, you need to make the differential inductor L1 and the output transformer T1. We wind the differential inductor L1 on a ferrite ring with high magnetic permeability and a size of Ø7x4x2 mm. Before winding, it is desirable to dull the sharp edges of the ferrite ring with sandpaper, and then, after degreasing the ferrite ring with alcohol or acetone, cover it with varnish or glue and dry it well. Both windings of the inductor contain 12 turns of AWG30 enameled winding wire. The winding method is shown in Fig.8.
Transformer T1 is wound on a ferrite ring with a size of Ø10x6x4 mm and a magnetic permeability of the material of at least 400. The sharp edges of the ferrite ring should also be dulled with sandpaper. Windings I and II contain 24 turns of AWG33 enameled winding wire, winding III – 20 turns of the same wire. The winding of the transformer T1 is performed as follows. First, 24 turns are evenly wound with a triple-twisted winding wire (see photo in Fig. 9), and then find the secondary winding with an ohmmeter. The other two windings make up the primary winding of transformer T1. The midpoint of the primary winding is obtained by connecting the end of one of these two windings to the beginning of the other.
Now we proceed to the installation of parts on the board. First, screw the bracket with the antenna connector. This must be done in order to then better see where to install the remaining elements, since the space occupied by the bracket can no longer be used for this. The remaining elements are installed approximately as shown in Fig.10.
After installing the elements in accordance with the schematic diagram, we solder the connections, but on the other side of the printed circuit board, approximately as in Fig.11.
You need to be especially careful when soldering the outputs of JFETs VT1 and VT2. Fig.12 shows the pinout of JFETs of the 2N4416A type when viewed from the terminal side. The output “Case” not indicated in the schematic diagram should be connected to the GND wire of the circuit.
The next stage is winding the contour coils of the ferrite magnetic antenna WA1:(I-II) and the symmetrical coupling coil WA1:(III-IV). First, we release both frames removed from the ferrite antenna of the radio receiver from the winding wire. Then, from a longer 11-section frame, we carefully saw off two sections with a hacksaw. As a result, we get two identical 8-section frames for loop coils and two sections for a coupling coil. Loop coils WA1:I and WA1:II contain 64 turns of AWG33 enameled winding wire, wound with 8 turns in each section. For winding, you can use the same wire that was first removed from these frames. It is convenient to fasten the ends of the windings with a thin cotton thread. The winding should be carried out in such a way that after installing the loop coils on the ferrite rod, the winding of one coil, as it were, continued winding the other.
To wind a symmetrical coupling coil WA1:(III-IV) we take the same wire. We twist it in half with a step of about 2-3 twists per centimeter of length and wind 24 turns of it on a 2-section section of the frame. Thus, each of the two halves of the coupling coil WA1:III and WA1:IV will contain 24 turns. To obtain the midpoint connected then to the GND wire, we connect the end of one half of the coupling coil to the beginning of the other. The result of all these winding operations can be seen in Fig.13.
We put the wound coils on a ferrite rod and install a ferrite rod on the board as shown in Fig.14.
The ferrite rod should be installed symmetrically relative to the board. The coupling coil is installed exactly in the middle of the ferrite rod, and the loop coils are installed at its ends, but also symmetrically relative to the middle of the ferrite rod. So that the ferrite rod does not slip in the holders and the coil frames do not hang on it, the fastening of the rod and frames should be sealed with a thick thread, preferably waxed, laid along the ferrite rod and passed through the coil frames and holders. We fix the coupling coil on the rod with ceresin or paraffin and solder its leads directly to the GND wire and the gates of JFETs VT1 and VT2. We also directly solder the extreme terminals of the loop coils to the stator terminals of the tuning capacitors C1 and C2.
Now let’s prepare the front panel with the tuning scale for assembly. First of all, we drill three mounting holes as shown in Fig.15. Then, on the reverse side of the panel, in its lower half, we glue the VD1 and VD2 scale backlight LEDs with epoxy glue (see on the right in Fig.15). The LED lenses should rest against the end of the plexiglass from which the round tuning scale is made. The LEDs must be facing each other with opposite leads, that is, cathode to anode.
After the epoxy glue has completely hardened, we solder the VD1 and VD2 LEDs in accordance with the schematic diagram. Then we assemble the vernier mechanism again, but with the difference that we now put the tuning wheel on another axis, away from the ferrite rod with contour coils. The view of the board with the assembled vernier mechanism before attaching the front panel with the tuning scale is shown in Fig.16.
The front panel is mounted on three standoffs 15 mm high. A general view of the assembled anti-noise ferrite rod antenna “Olusha-T” after its assembly is shown in Fig.17. To prevent the countersunk screws that screw on the front panel from standing out against its background, they can be painted over with black nitro paint.
With proper soldering of all connections, the antenna starts working immediately after turning on the power. It is only required to adjust the symmetry of the “shoulders” of the oscillatory circuit of the ferrite rod magnetic antenna and harmonize its resonant frequency with the position of the arrow on the tuning scale.
An active ferrite rod magnetic antenna made in this way can be connected to any radio receiver that has contact sockets for an external antenna. It can be either an old tube radio receiver from among those that did not yet include a ferrite magnetic antenna for the medium wave range, or a modern music center. Of course, such a music center is more convenient, since its digital scale allows you to immediately fine-tune to the desired frequency. These very musical centers for working on medium waves are equipped with a kind of simple, but inefficient non-resonant loop antenna (see photo in the title of the article). The anti-noise ferrite rod antenna “Olusha-T” antenna also works well with the popular DEGEN DE-1103 radio receiver, especially in winter, when long-range transmission in the medium wave range is much better.
Below is a photo of the active ferrite rod magnetic anti-noise antenna “Olusha-T” with the backlight of the scale in the evening, when, in fact, the air on medium waves begins to “come to life”.
Copyright © Sergii Zadorozhnyi, 2008
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